1. Field of the Invention
This invention relates to a method and an apparatus for roll forming a flat tube having a central reinforcement used for a heat exchanger or the like, and the flat tube.
2. Description of the Related Art
A flat tube for passing a heat exchange fluid is used for heat exchangers such as a radiator or a hot-water heater for automotive vehicles. Such a flat tube is brazed to a container (tank or the like) of the heat exchanger. A flat tube having an θ-shaped section with a central reinforcement is used for a wide heat exchanger. A method of roll forming the flat tube having the θ-shaped section from a single continuous plate material is disclosed in Japanese Unexamined Patent Publications Nos. 10-85877 and 2004-9087 filed by the same applicant as the applicant of the present invention.
The reinforcement of the flat tube is formed of perpendicular L-shaped bends and a folded rib formed of mountain-shaped folded-back portions, and therefore a very small depression is formed on the outer surface contour by the roundness at each bend of the plate material. This depression, though very small, results in a shortage of solder material used to braze the flat tube to the container of the heat exchanger and is required to be reduced as far as possible.
Especially, according to the roll forming method disclosed in Japanese Unexamined Patent Publications Nos. 10-85877 and 2004-9087, the depression generated at the base of the folded rib is reduced by upsetting the folded rib. In order to obtain a brazed joint high in reliability, it is desirable to further reduce such a depression.